In the field of drilling in the exploration for oil and gas, an important component is that of the formulation of drilling muds. Drilling muds are the fluids which are used to maintain pressure, cool drill bits and lift cuttings from the holes and vary in composition over a wide spectrum. Generally, drilling muds are based on aqueous formulations or oil-based formulations.
A conventional oil-based drilling mud formulation is comprised of basically the following ingredients: oil (generally No. 2 diesel fuel), emulsifying agents (alkaline soaps and fatty acids), wetting agents (dodecylbenzene sulfonate), water, barite or barium sulfate, (weighting agent), asbestos (employed as viscosification agent) and/or, amine-treated clays (also as viscosification agent).
The above combination of ingredients is generally formulated to possess various weights based primarily on amount of barite added. For example, a typical drilling mud can vary in specific gravity from a range of about 7 pounds per gallon up to 17 pounds per gallon or even greater. This variation in specific gravity is primarily controlled by the amount of barite added. The above formulations perform adequately in a number of applications, primarily those where the use of oil-based drilling muds is dictated by the lack of stability of the formation in which drilling is taking place. For example, in various types of shale formation, the use of conventional water-based muds can result in a deterioration and collapse of the shale formation. The use of the oil-based formulations circumvents this problem. However, it is observed that the current oil-based drilling muds have some significant disadvantages. One disadvantage is that the incorporation of asbestos or asbestos fines can incur significant health problems, both during the mud formulation and potentially during the subsequent use of such formulations. Therefore, it is desirable to eliminate the use of asbestos completely in such drilling muds. On the other hand, the use of substitutes for asbestos in this application has heretofore not been particularly successful in that the resulting viscosification agents must maintain adequate viscosities under the drilling conditions which can involve high temperature and high shear conditions.
There has been a substantial need for a drilling fluid which would exhibit good performance at high temperature in water sensitive formation. Past experience has shown that oil-based drilling fluids can provide good performance in water sensitive formations, and the state of the art systems can perform well up to about 350.degree. F. Typically, in such formations, the failure of the viscosities in current muds is circumvented by the addition of more viscosifier during the circulation of the drilling mud. While this solution is adequate at moderate temperatures, when much higher temperatures are encountered (example: geothermal wells or natural gas wells) the degradation of the viscosifier can be so rapid that the additional costs for a viscosifier can be uneconomical. There is a need, therefore, for drilling fluids which can maintain their viscosity and gel strength to temperatures up to and exceeding 400.degree. F. These needs are not adequately met by the current drilling fluids, even with the oil-based drilling muds often employed.
This invention describes an approach to viscosification of oil-based drilling muds which permits the substitution of powdered sulfonated thermoplastic polymers having a particle size range of about 0.25 to 25 .mu.m for asbestos fines and amine clays. The resulting polymer-modified drilling muds display improved low temperature rheological properties which include improved gel strength at up to temperatures of 300.degree. F. and higher, based on tests conducted for 16 hours at such temperatures.
The types of sulfonated thermoplastic polymers that are envisioned in the present invention include copolymers of sodium styrene sulfonate monomer with styrene vinyl toluene or t-butyl styrene monomer. These polymers possess suitable solubilities in the drilling mud environment.
In U.S. Ser. No. 292,233 Aug. 12, 1981, a high temperature drilling mud was formulated by the incorporation of a powdered sulfonated polystyrene directly into the drilling mud. The instant application differs from U.S. Ser. No. 292,233 filed Aug. 12, 1981 in that the powdered sulfonated polystyrene of the instant invention has a particle size of about 0.25 to 2.5 .mu.m, whereas the powdered polystyrene of U.S. Ser. No. 292,233, has a particle size of about 25 to 250 .mu.m and is used to formulate the drilling mud. Quite unexpectedly, the use of a sulfonated polystyrene having a substantially reduced particle size range in forming the drilling mud, results in a drilling mud having improved temperature rheological properties.
The instant invention will describe materials that provide improved and excellent gel strength at temperatures up to 400.degree. F. and may be effective at even higher temperatures.
The preferred species of the instant invention is a free-flowing, non-agglomerating powder of lightly sulfonated polystyrene having a particle size range of about 0.25 to 2.5 .mu.m with a sulfonate level of about 5 to about 100 meq. of sulfonate groups per 100 grams of sulfonated polymer. It is believed that this polymer is not soluble in either the hydrocarbon phase or the aqueous phase of a formulated mud and; therefore, the polymer is probably located at the interface. As a consequence, this polymer is an extremely effective viscosifier even at quite low levels.
A second facet of the instant invention relates to the use of these materials in formulations which employ high concentrations of salt in the aqueous phase. The powdered sulfonated polystyrenes (SPS) which are the preferred embodiment of this invention, lose some of their efficacy in salt water. It has been found that the combination of a suitable non-ionic emulsifier with the latex of the SPS gives formulations which are effective with salt water. Therefore, these systems give formulations which perform well at temperatures up to 300.degree. F. and in the presence of salt water phases, which is a highly desired objective in the drilling fluids industry.